SN75186

QUADRUPLE DRIVER/RECEIVER WITH LOOPBACK

SLLS068C – FEBRUARY 1990 – REVISED MAY 1995

11

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

In normal operation, the SN75186 acts as four independent drivers and receivers; the loopback mode is held off by

keeping logic inputs LB high. Taking a particular LB input low activates the loopback mode in the corresponding

driver/receiver pair. This causes the output from that driver to be fed back to the input of its receiver through dedicated

internal loopback circuitry. Data from the receiver output can then be compared, by a communication system, with

the data transmitted to the driver to determine if the functional operation of the driver and receiver together is correct.

In the loopback mode, external data at the input of the receiver is ignored and the driver does not transmit data onto

the line. Extraneous data is prevented internally from being sent by the driver in the loopback mode by clamping its

output to a level below the maximum interface voltage, – 5 V, or the EIA / TIA-232-E marking state. Below this marking

level, a reduced 1.5-V output amplitude is used at the driver output. This signal is detected by an on-chip loopback

comparator and fed to the input stage of the receiver to complete the loop.

Line faults external to the SN75186 are detected in addition to device failures. These line faults include short circuits

mode should be entered only when the driver output is low, that is, the marking state of EIA / TIA-232-E. Loopback

should not be entered when the driver output is in a high state as this may cause a low-level, nondamaging oscillation

at the driver output.

When in the loopback mode, approximately 95% of the SN75186 circuit is functionally checked. There exists some

low probability of fault mechanisms in circuitry not being checked in the loopback mode. To reduce the chances of

undetected failure, the unchecked circuitry has been designed to be more robust than that within the loopback test

loop. The areas where special attention has been paid are the receiver input potential divider and resistors, the driver

output blocking diode (D1), and parts of the driver clamp circuit.

Protection of the SN75186 is achieved by means of driver output current limits and a thermal trip. Although this device

can withstand

±30 V at its receiver input, package thermal dissipation limitations have to be taken into consideration

if more than one receiver is connected simultaneously. This is due to the possible dissipation in the 3-k

Ω minimum

input resistors, which is not under the control of the thermal trip. Although the supply current is higher in the loopback

mode than in normal operation, the total power dissipation is not sufficient under normal worst-case conditions (of

circuitry to trip.

If the SN75186 goes into thermal trip, the output of the driver goes to a high-impedance state and the receiver output

is held in a logic-high marking state. Both driver and receiver outputs maintain a marking state and do not allow

indeterminate conditions to exist.

The standards specify a minimum driver output resistance to ground of 300

Ω when the device is powered off. To fully

comply with EIA / TIA-232-E power-off fault conditions, many drivers need diodes in series with each supply voltage

to prevent reverse current flow and driver damage. The SN75186 overcomes this need by providing a

high-impedance driver output of typically 5 k

Ω under power-off conditions through the use of the equivalent of these

series diodes in the driver output circuit.